Unstable resonators (URs) have been used for years for high power lasers due to their large mode volumes. In the 1970s and early 1980s, they were applied to semiconductor lasers with mixed results. Recently there has been renewed interest in the use of unstable resonators for diode lasers. Advances in material uniformity and facet etching techniques have realized unstable resonator diode lasers that show good agreement between theory and experiment.
UR's for diode lasers are unique among UR's for several reasons. They are usually solid resonators whose outputs are taken through one of the end mirrors of its resonant cavity, and so the output beam is defocused by refraction as it leaves the resonator. Such refraction is acceptable because it may be refocused externally.
Due to their high gain, UR diode lasers are relatively short compared with their width, yielding high numerical aperture operation. The combination of these two factors with the use of circular reflective mirrors can lead to a significant spherical aberration in the output beam. However, because the etched facets of UR diode lasers are lithographically formed, the mirrors are not restricted to circular segments.
In principle, any mirror shape is possible. While the shape of the mirror determines both the shape of the mode and the effect of refraction through the output facet (hereinafter referred to as a mirror or reflector), it should be possible to construct a mirror shape that will yield an arbitrary mode pattern in the resonator, or equivalently, an arbitrary field pattern outside the resonator.